1. Field of the Invention
This invention is in the field of reclaiming foundry sand for reuse after contamination by metals, binders, and the like. The invention is concerned with treating the contaminated foundry sand sequentially in a magnetic separating zone, a thermal treatment zone, and a comminution zone from which the reclaimed sand particles are recovered.
2. Description of the Prior Art
Used foundry sand represents a mixture of more or less large agglomerates consisting of silica sand grains, dust, binding agents, and metallic particles which may remain from the casting process. The binding agents usually exist in loose form in admixture with the sand, but they may also be partially adhered to the silica sand grains. Binding materials such as bentonite clay, synthetic resins based on phenols or resols, water glass, and the like, are frequently present as binding agents.
The substantial costs involved in the preparation of sand for foundry use as well as the transporting and disposition of used sand, particularly in view of the requirements of keeping ground water clean, make a reclaiming of used foundry sand economically desirable. The reuse of the old sand, however, must be accompanied by processing which is directed toward the removal of contaminants from the sand which would interfere with the use of the sand in a mold making process.
For example, iron particles or other ferromagnetic particles tend to sinter together during the casting process with the silica sand grains and must therefore be removed completely along with fine grain components and binding agents. In addition, a cleaning device which is used to break up the sand agglomerates from the binding agent must be cleanly removed from the silica grains without, at the same time, causing excessive fracturing of the grains which would provide an undesired amount of fine grains.
The grinding of silica sand generally involves a significant wear on the grinding tools, particularly impact pulverizers and vibration grinding mills so that the economic feasibility of such recovery methods is impaired. Selection of a suitable cleaning process is also rendered difficult by the fact that the wear during the grinding process also produces unavoidable iron fines which are harmful in the casting process, and which have to be removed by a further step. It would be possible to use vibration grinding mills which utilize iron-free linings as well as appropriate grinding bodies, but this involves relatively high capital costs which limit the economic feasibility of the reclaiming process.